The only reason they ordered a mixed fleet was because neither manufacturer had enough slots to meet the demand for all the planes they needed solely... You can sort of say both sides were in the room when they worked out a deal..

"This new CSeries jet, which offers 110-seat and 135-seat versions, competes with the Boeing 737 Next Generation 737-600, 737-700, Airbus A318, Airbus A319, and Embraer 195. Bombardier claims the CSeries will burn 20% less fuel per trip than these competitors."

Remember you don't want an evacuation and the plane isn't secured and properly configured for it. Its best to avoid setting off the slides if a safer solution is possible because setting them off is costly and people usually end up getting hurt going down them. In this case they still ended up evacuating but everyone was on the same page when it happened. The Asiana 777 flight is kinda a different story because it was a full blown crash. Sometimes its just safer to stay in the plane then to go out and have people wander around an active airport because remember at that point the airport movements had still not come to a halt.